Sugar cell responses to lactose and sucrose in labellar and tarsal taste hairs of Musca domestica

Receptor cell responses in the largest labellar (LL) and tarsal (D) taste hairs of the housefly Musca domestica were investigated electrophysiologically using the tip-recording technique. In LL hairs, test series with lactose in concentrations of 12.5-400 mmol.l(-1) yielded a threshold concentration around 12 mmol.l(-1) and a calculated concentration eliciting half-maximal response of around 40 mmol.l(-1), the maximal response varying between 18 and 30 impulses/300 ms. D hairs are more sensitive towards lactose, indicated by a slightly lower threshold and a by 60% higher response to 400 mmol.l(-1) lactose. The high variation in the relative stimulating effectiveness of lactose and sucrose and experiments with sugar mixtures imply that these sugars bind to different receptor sites without noticeable cross affinity. A comparison of the concentration response characteristics for sucrose and lactose in LL and D hairs suggests that sucrose can combine with more than one site type, expressed in different proportions in both hair types. Results obtained with p-nitrophenyl-B-galactoside as stimulus indicate that a P-galactoside link is not sufficient for a substance to interact specifically with the lactose binding site. The exceptional lactose sensitivity of the sugar cell in M. domestica is discussed in the context of food acquirement and digestion.